1. Field of the Invention
The present invention relates generally to a method of adjusting the position of a plurality of magnetic heads mounted in a magnetic recording and/or reproducing apparatus which records or reproduces information to or from a recording medium in the form of a tape, and more particularly to a method of adjusting the height of each of the magnetic heads securely mounted on a rotary cylinder, which rotates inside the apparatus during recording or reproduction.
2. Description of the Prior Art
The accuracy required for properly positioning the magnetic heads in the magnetic recording and/or reproducing apparatus is an important factor to determine a track pattern to be recorded on the recording medium.
For example, in home video cassette recorders which are in wide practical use today, two pairs of magnetic heads are secured to a rotary cylinder substantially symmetrically on opposite sides thereof with respect to the center of rotation. The two magnetic heads successively obliquely scan a magnetic tape during recording or reproduction of information such as, for example, video or audio signal information. The pitch of tracks to be actually recorded on the magnetic tape is generally very small, i.e. about 60 .mu.m for a two-hour mode and of about 20 .mu.m for a six-hour mode.
If the two pairs of magnetic heads cannot scan these tracks with accuracy, it is impossible to accurately reproduce the recorded information, resulting in a considerable reduction in quality of reproduced images. To prevent such reduction, the scanning pitch of the magnetic heads is required to be in good agreement with the track pitch. One of important factors required to determine the scanning pitch is the height of the magnetic heads.
As shown in FIG. 1, the head height includes an absolute height Ha and a relative height Hr. The absolute height Ha is the distance between a reference plane 35 of a lower cylinder 31 and an lower end of a gap of a head chip 30 securely mounted on an upper cylinder 32 via a head base 33.
A conventional head height adjustment method is hereinafter discussed with reference to FIG. 1.
Initially, rough adjustments are carried out after a spacer 39 having a predetermined thickness has been inserted between the upper cylinder 32 and the head base 33 to which the head chip 30 has been bonded. Then, while the head height is being observed using a CCD camera 36 and a monitor 37, an adjusting vise or minute screw 40 in abutment with a portion of the head base 33 is screwed down by a required amount in a direction shown by an arrow 110, thereby causing partial deformation of the head base 33 and adjusting the head height to an intended height.
The principle of the conventional adjustment method with the use of the minute screw 40 is to utilize the deformation of the head base caused by the biasing force of the adjusting screw within the elastic limit thereof. In this case, if the extent to which the screw is threaded varies due to mechanical vibrations or temperature changes, the extent to which the head base 33 is deformed varies in proportion thereto, resulting in a change in head height.
Actual VTRs include factors such as, for example, cylinder rotation, tape travelling, and the like, which may cause vibrations, and temperature changes are not avoidable which are generally caused by changes with time in the amount of heat produced by various vibration sources or changes in environmental circumstances.
At present, an inhibitor (resin which sets at room temperatures) for preventing screw loosening is generally used to minimize the influence from these variation factors. The effect of this inhibitor is not sufficient to completely prevent the screw loosening, and the head height has hitherto occasionally changed in units of several microns.
In applications where a hydraulic bearing is utilized as a bearing for the rotary cylinder, the position of the rotatably arranged upper cylinder relative to the stationary lower cylinder greatly varies according to whether a rotary shaft 100 of the upper cylinder is rotating. Accordingly, in the conventional method wherein the head height is adjusted under a static state, it is difficult to maintain the accuracy of the head height during cylinder rotation. Eventually, adjustment efforts have no choice but to generally depend upon trial and error.
In order to overcome the problems inherent in the head height adjustment by the use of the minute adjusting screw, Japanese laid-open patent publication No. 4-310613 discloses a head height adjustment method wherein partial plastic deformation is caused by irradiating a laser beam to a head base carrying magnetic heads. According to this publication, the adjustable speed of the cylinder depends upon the accuracy of irradiation timing at which the laser beam is irradiated or the laser energy required to cause the deformation. Because of this, adjustments cannot always be carried out at a rated speed (the number of rotation of the cylinder at the time actual recording or reproduction is carried out).